In a gas system such as a fuel cell system that generates power by using fuel gas such as hydrogen gas as fuel, odorant is added to the fuel gas for detection of leakage of fuel gas. This enables a person to know leakage of fuel gas by using his/her olfactory sense to detect odorant that leaks along with the fuel gas.
However, odorant may sometimes interfere with function to be fulfilled by a gas system. For example, in a fuel cell system, if fuel gas (hydrogen) that contains much odorant is supplied to a fuel cell, it may result in a decrease in generating efficiency of the fuel cell. A technique is therefore disclosed that maintains concentration of odorant in fuel gas to be supplied to a fuel cell within a predetermined range (see Japanese Patent Application Laid-Open No. 2004-111167). In this technique, concentration of odorant to be supplied to a fuel cell can be maintained within a predetermined range by unit of an odorant concentration control unit that is provided upstream of the fuel cell and a circulation channel for its fuel gas. Other conventional techniques are also disclosed in Japanese Patent Application Laid-Open No. H10-115587 and Japanese Patent Application Laid-Open No. 2002-29701.